Until recent years, the majority of indicating lights had the disadvantage that they provided a field of illumination which was not at all homogeneous, despite various devices for spreading the light which are in use at the present time, and which generally consist of billets or toroids incorporated in the cover glass or in an optical plate, or screen, of the indicating light. This disadvantage, which is in general tolerable from the point of view of conforming with the law, is a definite drawback from the aesthetic point of view, while homogeneity of the fields of illumination of the indicating lights is a matter which is now considered to be of increasing importance.
It is known from the French published patent document No. FR 2 614 969A to provide an indicating light with a homogeneous field of illumination. This indicating light includes, within a housing, an optical screen which is transparent or translucent and which is referred to as a "bonnet", being interposed between a light source and an essentially flat optical screen or plate which is set at right angles to the optical axis of the indicating light. The particular geometrical configuration of the bonnet ensures that the surface density of the light flux arriving on the flat optical plate is substantially constant over the whole surface of the optical plate. Finally, the surface of the optical plate is provided with striations which align the light rays received from the bonnet, in order to transmit outside the indicating light a beam consisting of rays parallel to the optical axis of the light itself.
The homogeneity of lighting provided by such an indicating light thus provides an aesthetic advantage by comparison with previous arrangements. However, the striations which are carried by the flat optical plate do not enable light to penetrate into the indicating light, and consequently an observer looking at the light from outside will only see an opaque surface. Now, a current tendency in matters of styling for automobile indicating lights lies in the design of such lights so that they have a so-called "glassy" appearance, smooth and transparent, such that what an outside observer is able to perceive (whether it be what he really sees or whether it be an illusion) is a certain amount of depth in the hollow interior or cavity of the indicating light.